The Vanishing Ice and the Rising Sea
A coastal town measures sea level with a tide gauge and finds it has risen 18 cm over 60 years, about 3 mm per year. A nearby glacier on land has retreated 400 m in the same period, and satellite data show the floating sea ice around the coast has thinned. A town council member argues that the melting sea ice is the main cause of the local sea-level rise and proposes building a seawall 1 m high. A scientist notes that the glacier sits on land and drains into the same bay. The town has a budget for only one major project and wants the most defensible decision.
1. Distinguish the contributions of the floating sea ice and the land glacier to local sea level, and explain which is the larger concern.
Read the worked answer
Floating sea ice is already displacing water, so when it melts it adds essentially no new water volume to the ocean; its thinning does not raise sea level appreciably. The land-based glacier is different: its ice is stored on land, and meltwater running into the bay adds water that was not previously in the ocean, so it contributes directly to sea-level rise. The 18 cm rise over 60 years is therefore better explained by land-ice melt, thermal expansion of warming seawater, and possibly local subsidence, not by sea-ice loss. The council member has confused two reservoirs. A defensible answer identifies the land glacier and ocean warming as the real drivers and treats the sea-ice argument as a misconception, while acknowledging that other local factors such as groundwater withdrawal could also contribute.
2. Given a fixed budget, recommend how the town should decide between the seawall and other options, and state what additional measurement would most improve the decision.
Read the worked answer
The town should first determine whether the 3 mm per year rise is accelerating and how much comes from local subsidence versus global ocean change, because a seawall designed for 1 m may be over- or under-built. The most useful additional measurement is a long-run local record combining the tide gauge with land-motion data, such as continuous GPS at the gauge, to separate sea-level rise from the land sinking. If subsidence dominates, managing groundwater or relocating infrastructure may be more effective than a wall. If the rise is mostly ocean-driven and accelerating, a seawall with a design allowance for future rise is reasonable. The recommendation should be conditional: gather the separating measurement, then choose the project that addresses the dominant cause, rather than committing the whole budget to a wall built on the sea-ice misconception.